The field of this invention is downhole cleanup of casing and liners and more particularly after cementing and before completion.
The cementing process is known to leave debris such as cement lumps, rocks, and congealed mud in the casing or liner. Other debris can be suspended in the mud and it can include oxidation lumps scale, slivers, shavings and burrs. A variety of well cleaning tools have been developed particularly to dislodge such debris from the casing or liner walls. Jet tools are used to blow such debris loose. A variety of casing scrapers and brushes have been developed to accomplish the same purpose. These tools have more recently been combined with additional tools to filter the downhole fluid and capture the debris therein for removal to the surface.
One such debris filtering tool is described in UK Application 2 335 687 and is called the Well Patroller, a trademark of the owner Specialised Petroleum Services of Aberdeen, Scotland. This device generally features a wiper cup that rides the inside of the casing. The cup prevents flow around a mandrel. As the tool is lowered, flow is directed through a plurality of ball check valves into an annular space behind a screen and out though the center of the cup and around the mandrel. In this embodiment, no filtration occurs as the tool is inserted and the cup wipes the casing wall. When the tool is brought out of the wellbore, the ball check valves close and fluid above the cup is directed to the annular space inside the filter and out through the filter. The annular space acts as a reservoir for debris retained by the filter. If the filter clogs pressure can be built up to blow a bypass rupture disc, or, in some embodiments to simply shear screws and blow the cup off the mandrel. There are shortcomings in this design. The most significant is that the opening size in the check valves is small and is prone to plugging with debris. When running in the Well Patroller, downhole progress is stopped every 90 feet or so as another stand of tubulars is added at the surface. During these times the fluid flow through the tool stops and debris suspended in the fluid will settle to the bottom of the tool. The debris will eventually accumulate to the point which the ball check valves can not open. Once fluid can not pass though the check valves, the annular restriction at the top of the tool will force the annular fluid to pass through the screen. Any debris in the fluid will not be able to pass through the screen. When the tool is pulled out of the well, the debris will be left in the well. The Well Patroller tool is used in conjunction with a separate tool to scrape debris off the inside casing wall. The wiper cup""s purpose, in this tool, is to divert flow as opposed to scraping or swabbing the inner casing wall.
Other debris removal tools are shown in UK Application 2 335 218; U.S. Pat. Nos. 4,515,212 and 5,330,003. The tool in UK Application 2 335 218 requires forced circulation through a plurality of eductors coupled with a deflector for the induced flow to encourage solids to drop into an annular space. Boot baskets, such as those made by Tri-State Oil Tools Industries Inc., now a part of Baker Hughes Incorporated featured an annular space defined between a solid basket and a mandrel. Solids were capable of being captured on the trip downhole solely due to the velocity decrease as the flow emerged above the boot so that solids could drop into the annular space between the mandrel and the boot. Since the boot was solid, no meaningful capture of solids occurred on the trip out of the hole.
One of the objects of the present invention is to eliminate or, at least minimize, the shortcomings of the Well Patroller device and the other tools previously used to filter downhole debris. The objective is addressed by providing an improved open area in the valving to reduce the potential problems from plugging. Another feature is the retractable flow diverter which allows rapid insertion into the wellbore, and provides easy passage of suspended debris past the tool. Yet another feature improves the valve structure in this application to get away from spring loaded balls which can create maintenance concerns. These and other advantages of the present invention will be more readily apparent to those skilled in the art from a review of the preferred embodiment which appears below.
A downhole debris filtering apparatus is disclosed. In a preferred embodiment, the flow diverter is retracted as the tool is run in to provide easy passage for debris. Settling velocity is the rate at which debris will fall through the fluid. Settling velocity is dependent on the density of the fluid and the debris density. If the fluid velocity exceeds the settling velocity, the debris will rise. The restricted annular area outside the screens raises the fluid velocity. Above the cup sleeve, the annular area increases significantly and the fluid velocity is reduce to below the settling velocity to allow debris to settle to the bottom of the tool. On run in, the fluid merely bypasses on the outside of the filter. Large debris will settle into the tool as described. When pulled out of the hole the swab cup/flow diverter is actuated into casing or liner wall contact and a large passage is opened to allow flow though the filter. Small debris that has risen above the tool will be captured as the fluid is filtered through the screen. Another embodiment with a non-retractable cup uses a sliding sleeve valve with a large open area which promotes free flow and minimizes fouling from deposited debris.